Progressive spectacle lenses and the individual optimization of aberrations of progressive spectacle lenses were described in many publications. For most applications, the general aim was the best improvement of the quality of image formation, i.e. reducing aberrations as far as possible. The highest qualities are presently achievable with spectacle lenses that are optimized individually for a particular wearer with the individual preferences of use being taken into account. For these spectacle lenses, the design of the spectacle lenses (i.e. the distribution of the effective power and astigmatism) are adapted to the individual needs of the wearer. When minimizing the aberrations of a spectacle lens, all individual prescription data (spherical power, cylindrical power, cylinder axis, prism, prism base, addition) as well as the individual data of wear (vertex distance HSA, pantoscopic angle, face form angle, interpupillary distance) and additionally, even the object distance in the reference points are taken into account.
When adapting the dioptric power to the ordered prescription of the wearer, conventionally only two different procedures were available:
1) Conventional Prescription Surface
                In this procedure, a surface with a simple mathematical representation, such as a toric surface, is calculated for the rear surface (in spectacle lenses based on conventional blanks) or as a surface to be superimposed to a progressive base surface (in simple progressive additional lenses with a progressive rear surface fabricated according to free-form technology) such that the prescribed effect is met in the optical reference point. Available individual data of the wearer (e.g. position of wear) may be taken into account. No surface optimization is carried out.2) Individually Optimized Prescription Surface        An optimization of the entire surface is carried out while the individual data of the prescription as well as data of the position of wear are taken into account. As far as the individual data of the position of wear are available, these data are taken into account for the optimization. Otherwise standardized data are used. In any case, all available parameters are used in order to achieve the best possible spectacle lens. The calculated and optimized surface is typically the rear surface, while the front surface may be a single vision surface (i.e. spheric, aspheric, or toric surface) or a progressive surface.        
All conventionally proposed methods of optimization aim at the best possible improvement of the quality of image formation through consideration of all available parameters. Since the launch of the product “Impression” by Rodenstock in the year 2000, more and more companies try to make use of the so-called free-form technology in own products. Not least because of the great success of “Impression”, the spectacle lenses with a spherical front surface and a progressive-astigmatic rear surface are considered as first grade for correcting presbyopia. However, many of those spectacle lenses fabricated with the free-form technology are not individually optimized, but their optical correction quality is similar to lenses based on conventional blanks with a progressive front surface (base curve) and a spherical or toric prescription surface. The only difference is that for the optimization and fabrication of many of those conventional spectacle lenses fabricated with the free-form technology the simple prescription surface is superimposed to a progressive base surface. Similar to glasses based on conventional blanks, for each mean spherical power a progressive base surface is calculated and stored as a base design for each glass material, each addition, and each progression length in advance. The typical range of spherical power covered by a single base surface is about 3 dpt. Therefore, about 6 base designs are required in order to cover the normal range of the spherical power between about −10 dpt and +10 dpt.
For a product family with three progression lengths, five materials, and twelve additions (0.75 dpt to 3.5 dpt) at least 6*3*5*12=1080 base designs have to be calculated and stored in advance. Moreover, for positive lenses at least two (better three) base designs should be provided for different glass diameter, since the centre thickness of positive lenses depends on the diameter of the glass, and at higher surface curvature of the front surface the addition strong varies with the surface curvature. Therefore, a reasonable conventional approaches base on the free-form technology require about 1500 to 2000 base designs.
These base designs are calculated and stored on a computer in advance. Subsequently, a simple superposition surface is added to the progressive rear surface in order to meet the required prescription in the reference point. These glasses have implemented the progression on the rear surface (i.e. the eye side surface) and they are manufactured with the free-from technology. However, since no power-dependent optimization was carried out and since none of the individual parameter of wear are taken into account for the surface optimization, the optical quality, i.e. the quality of image formation, of these spectacle lenses compares to that of conventional lenses based on blanks. Therefore, these spectacle lenses also have the known deficiencies of the conventional blank products. Although manufacture of these lenses is equally costly as for progressive spectacle lenses that are fully individually optimized online, the optical properties may significantly deviate from the target value of the base design, particularly for astigmatic prescriptions and/or prismatic prescriptions and/or deviating individual parameters and/or extreme diameters of the glasses. Therefore, these spectacle lenses from the lower price range within the branded progressive lenses.
On the other hand, if the presently most sophisticated optimization process is applied (individual online optimization, flexible optimizer, scaling of the astigmatic target value, automatic design modification), only about 5 start designs are required to provide optimal image formation of the spectacle lens with any design for all additions, material, progression lengths, base curves, individual parameters of wear, and prescriptions. These spectacle lenses, therefore, represent the top-quality products for the spectacle wearer.